Modern inventory systems, such as those in warehouses, superstores, mail-order, and e-commerce warehouses and larger manufacturing facilities, use gantry storage—and retrieval (S&R) systems that require dedicated space and large capital investment or vertical pallet stacking on shelving served by manually driven fork trucks. The former provides fast, accurate responses to requests for inventory items but is a large integrated capital investment. On the other hand, manual forklift vertical storage systems can cause delays and backlogs in the process of responding to inventory requests, and furthermore require drivers and loaders and their salaries, benefits, and management burden.
Historically, inventory systems are based around high vertical storage using pallet stacks or pallet stacks on shelving units. This arrangement offers a compromise between easy access and 3 dimensional storage to reduce floor space. This type of storage is accessed by forklifts of various configurations. The typical way this type of 3 dimensional storage is automated is through gantry style S&R units that provide three dimensional access to the pallet storage locations along a fixed set of travels (row selection, perhaps through an automated conveyor; column selection with a pallet transport device moving along a preplaced rail, and vertical pallet retrieval or storage along an elevator lift mechanism).
The type of automation just described is expensive, requires rework of the entire storage/retrieval space, and is an all or nothing proposition—one cannot practically partially automate a space—it either is automated or remains manually retrieved with forklifts. Furmans, et al. (“Plug-and-Work material handling systems.” 2010 International Material Handling Research Colloquium, Milwaukee, USA. 2010) provides an overview of the current state of the art, and suggests modularizing S&R functionality as a means to improve it. As an illustrative example, the SmartRack and the KARIS Flexconveyor provide a computer controlled roller conveyor segment mounted on an autonomous material handling robot platform.
Solutions to the existing lack of flexibility in this field are addressed in various U.S. Patents assigned to Kiva Systems. In the Kiva system, the normal storage shelves with pallets stored at fixed positions is replaced by what Kiva refers to as inventory holders. These items are basically risers upon which pallets or other items (for instance shelves or work stations for package integration operators) can be placed so that they are up off of a flat transport surface normal the warehouse floor. The inventory holders are loaded at an incoming area, usually manually or via some kind of lift assist (forklift).
What Kiva refers to as a mobile unit—a small four wheeled mobile robot—moves under the inventory holder. Using a lift elevator attachment, the mobile unit lifts the inventory off of the support surface, and moves to the destination storage location. In many uses of the system, this could be any empty location over the warehouse floor that can be remembered and recalled by an inventory location database. At the destination location, the mobile unit lowers the inventory holder and is then schedule to execute its next pick-up and place operation. Retrieve works the same ways but in reverse. A mobile unit goes to the pick-up location in the storage array, lifts up the selected inventory holder, brings it back to a work cell or packing area, and places it down to await the next pick and place operation.
The advantages of Kiva's system is that storage area can be allocated as a dynamic buffer, and S&R performance can be optimized both by storage layout and by the number of mobile units acting in the system. By using standardized holders, the mobile units only need to implement a single and very simple lift elevator. By dividing the storage space into grids, and labeling at least some of the grids with location barcodes on the floor (Kiva calls them fiducials), centralized control of very simple mobile robots becomes feasible by block reservation and relocalization each time a unit passes over a barcode. Robot unit sensing devolves to being able to count distance and 90 degree turns and periodically detecting and locating a ground placed fiducial labeled so that the system can easily identify its location relative to that fiducial within the warehouse location grids. Path planning and collision avoidance is handled through the central controller by reserving the next grid any particular mobile unit plans to traverse, or waiting for that grid's currently occupying unit to move out of the grid (i.e. the requesting unit waiting for the grid reservation request to be satisfied or completed before moving into it).
The Kiva approach has several disadvantages. First, like a 3D gantry S&R unit, it is an all or nothing proposition—one cannot easily mix manual operated lift units with either of these automated concepts in a shared common area. Manual transport devices or people are not equipped with the sensors or controllers to reserve grids so are invisible to the central transport scheduling system. Kiva mobile units might be seen by people or manually controlled transports, but mobile units are not equipped with the sensing required for them to see the manually controlled assets. Therefore safety and collision avoidance cannot be assured in mixed operation. This means that in operations that uses conventional lift trucks or mix inventory storage with shopping (for instance the super stores like Home Depot or Sam's Club where product is stored in vertical shelving, placed there by lift trucks, but selected by people that walk around the aisles) cannot easily mix with the Kiva S&R system. Also Kiva is basically a 2 dimensional system that requires large ground footprints. In urban areas where land is more expensive, this might be too costly and low density.